1. Technical Field
The present invention relates to a method and apparatus for washing and dewatering a liquid suspension of solid biological pulp material having a wide range of solids concentrations, comprising a mechanical or pressurized distributor connected to a press washer having multiple nips and multiple washing zones.
2. Related Art
In industries engaging in the production of pulp from solid biological material, such as papermaking pulp from cellulosic fibers, it is known to wash liquid pulp suspensions in order to remove organic and inorganic impurities. Washing a liquid pulp suspension typically involves flushing the suspension with water or another liquid (washing) and then removing, or simultaneously removing, at least a part of the water in the suspension (dewatering). The dewatered liquid carries away impurities contained in the pulp suspension.
Four basic types of conventional pulp washers include: drum washers with vacuum filters; belt washers with filter belts or double wire presses; diffusers for displacement washing in a tower; and washer presses in which the pulp suspension is dewatered only to a certain solids concentration. The present invention relates to the field of washer presses.
Conventional washer presses typically comprise one or two rotatable, cylindrical drums having perforations or slots capable of receiving wash liquid. The drum or drums are arranged and mounted within a correspondingly-shaped housing. The sides of the drum housing, more commonly referred to as the vat, extend partially about the surface of the drum or drums. For example, in twin-drum configurations, the drums are positioned side-by-side within a single vat, which usually extends about 180 degrees of the circumference of each drum. Conventional washer presses also typically include a feed distributor (or feeder) located near the top of the vat for introducing the pulp suspension into the washer press at a drum inlet. The pulp suspension then moves within a trough adjacent to the drum.
Washing and/or dewatering a liquid pulp suspension in a washer press involves pressing the pulp suspension against at least one rotating cylindrical drum in order to separate some of the water and move this water out of the pulp suspension into the interior of the drum. As the pulp suspension is pressed against each drum, the water or other wash liquid is pressed through slots or perforations into the interior of the drum and a mat of pulp is formed and is typically scraped off by means of a scraper. The pulp is compressed within a trough formed between a pulp slide plate or the vat and the exterior surface of the drum. The width of the trough decreases and converges to form a narrow point, called a nip, such that the pulp suspension is compressed within the narrowing trough as it is moved by the rotation of the drum toward the nip.
Alternatively, the pulp suspension can be compressed mechanically by moving the position of the pulp slide plate to decrease the distance between the pulp slide plate and the exterior surface of the drum, or this method can be combined with other compression methods. The compression of the pulp suspension by a movable or fixed pulp slide plate (a movable pulp slide plate is described in WO2010/116026) causes a portion of water in the pulp suspension to be moved through the perforations in the drum into the interior of the drum.
In wash presses incorporating a nip and pulp slide plate, the pulp suspension is most effectively washed immediately after it has been compressed and dewatered. In general in the industry, a goal is to achieve about an 8% to 10% solids concentration level in the pulp suspension at the nip point before the suspension is washed. Although higher solids concentrations at the nip point, for example, up to 15%, would yield an overall increase in the amount of impurities removed from the pulp suspension, such concentration increases are difficult to achieve due to problems with clogging of the trough.
Attempted solutions to this problem have included the introduction of additional washing at various intervals along the trough between a first, or primary nip and a last, or final nip to increase the level of impurities removed (a single displacement wash zone). The introduction of wash water in a single displacement wash zone, however, does not yield optimal results because the wash water merely mixes with existing impurities. In addition, the introduction of wash water into a single displacement wash zone is less controlled if added to the pulp suspension after it has passed the area adjacent to the primary nip. An addition of water in this manner also leads to decreased dewatering of the suspension at the end of the cycle. Movable pulp slide plates have also been used to alleviate clogs; however, washing efficiency is still limited by the physical space within which the washing must take place as the suspension moves around the drum.
Accordingly, there is a need for an improved washing and dewatering method and apparatus, more particularly a washing and dewatering method and apparatus with improved efficiency that is capable of washing and dewatering a wide range of solids concentrations in liquid suspensions. There is also a need for an improved washing and dewatering method and apparatus that provides increased impurity removal and washing capacity. There is a further need for an improved washing and dewatering method and apparatus that provides either standard washing results using less water, or improved washing results using a standard amount of water. There is still a need for a washing and dewatering method and apparatus that will accommodate a range of solids concentrations for processing. It is to these and other needs that the present invention is directed.